Multi-channel array antennas in various systems have been used to digitally form antenna beams for the past few decades. By digitally forming the antenna beam, adaptive processing techniques can be implemented to shape beam patterns so that energy received in the direction of interference is minimized while energy received in the direction of the desired signal is maximized. This process is typically referred to as adaptive beam forming. Some examples of applications implementing adaptive beam forming techniques include radar, sonar, seismic, imaging, cellular, satellite, and global positioning systems.
Adaptive beam forming processes attenuate interference signals impinging on the antenna array by placing beam pattern nulls in the direction of the interference. However, the amount of attenuation is limited by the level of matching between the channels. A relatively recent method for achieving high levels of receiver channel matching employs the use of adaptive channel equalization. In such a process, analog signals are received by individual antennas, propagated through the receiving hardware of each channel, converted to digital signals, equalized, and finally combined to form the antenna beam. Therefore, any system employing adaptive beam forming can implement adaptive channel equalization techniques for receiver channel matching.
The receiver channel matching is accomplished digitally by inserting a tapped delay line filter in each channel, selecting a reference channel, and generating equalization tap weights to match the channels to the reference in both phase and amplitude. This process is performed for each system calibration cycle, which occurs just before receive data is captured and processed. The equalization is typically performed every system operational cycle, or dwell. Matching the channels in this manner improves the channel matching results and reduces the cost and complexity of receiver hardware by decreasing manufacturing tolerances. The level of matching achieved is determined by the number of filter taps, the signal to noise ratio (SNR) of the channels, and the reference channel characteristics. Given that the number of filter taps is limited by the processing throughput and the SNR is sufficient, improved matching can be achieved by intelligently selecting the reference channel. Previous implementations of this matching procedure chose an arbitrary, but properly functioning channel, as a reference during system initialization. The reference channel could then fail or degrade over time and greatly reduce the channel matching. One such method of channel equalization is described in U.S. Pat. No. 5,357,257. In that patent at page 7, lines 63-66, the inventor states that any operating channel can be chosen as the reference channel. The present invention improves upon this prior art by choosing a reference channel during system calibration cycles that produces the optimal, or near optimal, channel matching.